1. Field of the Invention
The present invention relates generally to impact printing mechanisms used in typewriters and printers and more particularly to a low cost impact printer mechanism which produces a low level of acoustic noise during operation.
2. Description of the Prior Art
Both typewriters and printers utilizing impact printing mechanisms often generate high levels of acoustic noise. There have been various solutions proposed to lower the noise generated by such printing mechanisms. It has, for example, been the practice in the typewriter and printer art to reduce noise by the use of platens having a reduced hardness. This solution has, however, been found to also reduce the print quality. Another practice has been to reduce the required impact velocity by increasing the effective or apparent mass of the hammer or anvil. Other examples of mechanisms embodying this practice are disclosed in U.S. Pat. Nos. 4,668,112, 4,681,469, 4,678,355, 4,737,043, 4,859,096, 4,867,584, and 4,874,265. These mechanisms typically include an additional mass which is remote from and coupled to the print hammer by a rigid connecting drive member. Another example is U.S. Pat. No. 1,561,450 which discloses a weight remote from the type bar that upon activation produces the necessary momentum to straighten a toggle and impart movement to the type element. An example of a mechanism in which the weight is mounted on a print hammer activated by a solenoid is the Model A-3250 electronic typewriter manufactured by Sharp Corporation.
In accordance with the present invention, a weighted print hammer is driven by a cam. The combination of a print drive cam and a continuously rotating motor to accelerate a single head print element for printing is disclosed in U.S. Pat. No. 4,359,287, but unlike the present device, the print hammer disclosed therein is not weighted.